1. Field of the Invention
This invention refers to modified heparins having antithrombotic activity by oral administration, a method to obtain said heparins and pharmaceutical compositions containing them.
Heparin is one of the best known natural substances used in therapy due to its unreplaceable anticlotting and antithrombotic activity.
Heparin is a complex heteropolysaccharide made by repeating disaccharide units. Each unit comprises a uronyl residue bound to glucosamine-O- and N-sulfate.
Uronic acids in the sequence are alternatively: D-glucuronic, L-iduronic, L-iduronyl-2-sulfate.
Glucosamine is sulfated in positions corresponding to the amino and 6-hydroxy groups. In some sequences also a peculiar glucosamine trisulfate is found, having an extra hydroxy group in position 3. This glucosamine trisulfate, although representing only a small fraction of the total glucosamine in the heparin molecule, is basically significant for the anticlotting activity.
On this basis, the natural heparin could be described under the following formula: ##STR1## where: R=H or --SO.sub.3.sup.-, m/n: 2:1, m+n=about 20 (average).
This formula shows that the only hydroxy group that is always free in the heparin molecule is the one in position 3 of the uronic acid.
The many different biological and pharmacological activities of heparin are the consequence of intricate and partially not well understood biomolecular mechanisms. Actually, we know that the anticlotting activity of heparin is related to its specific link with a protease pro-inhibitor known as antithrombine III (At-III). The specific site in the heparin for the bond formation with At-III is a pentasaccharide sequence having glucosamine tri-sulfate in its centre.
On the other hand, the heparin anticlotting activity is not strictly necessary and proportional to its antithrombotic activity.
On the contrary, in many instances it is desirable to reduce the anticlotting activity whenever this activity can be identified with a hemorrhagic effect. In fact, some modified heparins having low anticlotting activity show an optimal antithrombotic effect. These preparations are considered as a great improvement on normal heparins, as they act with a therapeutical action of paramount importance in the prophylaxis of thrombosis jointly with a lower risk of hemorrhage.
It is now accepted that the antithrombotic effect can be roughly evaluated in vitro through the assay of the specific factor X activated inhibition (anti-Xa activity).
Unfortunately, heparin is active only when administered by parenteral route and is not orally absorbed.
The low or null biodisponibility of the orally administered heparin hampers its use in long term therapies and prevents patients in "thrombogenic state" from keeping constant levels of anti-Xa activity.
Heparin is not orally absorbed essentially for 3 reasons:
(1) It is a high molecular weight polymer (12000-15000 D) and there are no enzymatic mechanisms in the digestive tract or in the bloodstream able to split heparin molecule.
(2) It is an extremely hydrophylic substance (partition coefficient in n-octanol/water less than 0.01).
(3) It is a highly ionizable compound.
2. Description of the Prior Art
The heparin controlled depolymerization can be obtained through various methods described in the literature: nitrous acid treatment, beta-elimination, peroxydolysis, atomic oxygen action, molecular sieve fractionation. Therefore, at present, it is quite simple to obtain heparin fractions having a mean molecular weight (MW) lower than the commercial heparin, showing high anti-Xa and low anticlotting activity (LMW Heparins), but depolymerization, per se, is not sufficient to obtain an oral heparin. In order to obtain by semi-synthesis new molecules of heparin derivates with high anti-Xa activity as well as the possibility of oral absorption, we have settled an original process which allows us, just in one step, both to substitute the sulfate group in the amino nitrogen of the heparin and to esterify specifically the hydroxy group in position 3 of the uronyl residue with the same acyl groups. Undoubtedly, this double substitution occurs if the hydroxy group has not been previously etherified or esterified.